The present invention relates to a slip suppressing device for detecting a slip state of driving wheels and suppressing a slip and a suspension control device for controlling a suspension state of the vehicle.
In an automobile (vehicle) which travels by rotating wheels by a driving force of an engine, when the driving force of the engine exceeds a frictional force between driving wheels and a road surface when the vehicle is immediately started or accelerated or travels on a snow road, the driving wheels slip. For example, in a rear-wheel driving vehicle, when an excessive slip occurs between rear wheels as the driving wheels and a road surface, the rear wheels make a behavior such as a skid, interfering with stable traveling, thus impairing travel stability of a vehicle.
In recent years, a technique for executing slip control (traction control) has been developed. In this technique, when driving wheels excessively slip, the rotating torque of the driving wheels is decreased to suppress the slip, thus realizing an efficient drive operation and travel stability. According to this traction control technique, a slip state of the driving wheels is detected on the basis of a difference in rotating speed between driven and driving wheels. When a slip ratio of the driving wheels exceeds a predetermined slip ratio, a brake or an accelerator is controlled to decrease the rotating torque of the driving wheels. In this manner, according to this traction control technique, the slip of the driving wheels can be effectively suppressed.
In a so-called front-engine, rear-drive vehicle in which a driving force of an engine is transmitted to rotate rear wheels through a propeller shaft arranged along a longitudinal direction of a vehicle and a differential device connected thereto, when the slip control is performed as described above and, in particular, when a slip suppressing control operation is performed by means of a brake, the rotating torque of driving wheels, i.e., rear wheels is forcibly suppressed by the braking operation of the brake. For this reason, a counterforce is generated in a direction opposite to the rotating direction of the rear wheels, and acts on the differential device.
The front and rear end portions of the differential device are supported at predetermined positions of a vehicle through a plurality of mount members. As a result, when a counterforce in a direction opposite to the rotating direction of the rear wheels acts on the differential device, an upward load acts on the front end portion of the differential device, and a downward load acts on the rear end portion. For this reason, a so-called "squat" phenomenon occurs wherein a vehicle body at the rear wheel side, i.e., a rear portion of the vehicle body becomes lower than a front portion of the vehicle body. The squat phenomenon considerably impairs comfort, thus making passengers uneasy.
For example, Japanese Utility Model Laid-Open (Kokai) No. 60-43413 discloses a technique for arranging a pair of left and right shock absorbers with air springs and a high-pressure air source connected to air chambers of the air springs of the pair of shock absorbers through communication paths. According to this prior art, a vehicle height adjusting device for adjusting a vehicle height by supplying/exhausting compressed air to/from the air chambers through the communication paths is proposed. However, the vehicle height adjusting device does not aim at preventing a so-called "squat" phenomenon of the rear portion of the vehicle body during slip control by a slip control means, which is recognized as a problem in the present invention.